Figure 2.9

Equinox day length. On the equinoxes, the subsolar point is at the equator, so the circle of illumination (which, as we have seen, is always 90° from the latitude of the subsolar point) passes through the poles. This position makes the length of the daylight side of the planet equal to the length of the night side of the planet. Therefore, any point on Earth, except the poles, will take 12 hours to rotate through the daylight side and 12 hours to rotate through the night side. At the poles, the half-disk of the Sun will trace the line of the horizon where the horizon is flat, such as over the North Pole’s sea ice.